This invention relates to a method of connecting metal leads with electrodes of a semiconductor device, and more particularly relates to such a method in which a plurality of leads are bonded at the same time to the semiconductor device and a metal lead therefore.
According to the recent progress in miniaturisation of electronic home-appliances and industrial equipment, it has been urgently required to make semiconductor devices such as IC, LSI, etc. small and thin. A silicon wafer on which the semiconductor devices are formed through steps of diffusion and making electrodes, is cut into a plurality of chips of unit semiconductor devices. The packaging is provided for each semiconductor device in such a way that aluminum electrodes provided at the peripheral portions of the chip can be connected with external terminals by leads for ease of handling and further the chip is protected from mechanical damage.
Among the hitherto known methods of packaging, the tape carrier method has been a excellent one. According to that method, on each electrode terminal of the semiconductor device is provided a metal multilayer called barrier metal and on the metal multilayer is provided a metal bump by electroplating. A plurality of metal leads supported on a polyimide tape are bonded at the same time to the metal bumps corresponding terminals (called lead gang bonding). This method can provide a high reliability of bonding, a small-sized, thin package and an easy manufacturing operation.
It has, however, shortcomings as follows:
The metal bump in the tape carrier method has a structure as shown in FIG. 1A. In this figure, 1 denotes a semiconductor substrate with a semiconductor device formed thereon. The substrate 1 is covered with a passivation layer 2 for protecting the semiconductor device. The passivation layer 2 has an opening so as to expose an electrode terminal 3 of the semiconductor device. On that electrode terminal 3 is deposited a barrier metal 4 which is composed of a multilayer of Cr--Cu, Ti--Pa, Ni--Cu, etc. and formed by a continuous deposition in a high vacuum. In such a metal layer, Cr, Ti and Ni provide good strength of the bonding to the electrode terminal 3. On the barrier metal 4 is formed a metal bump 5. It is formed by electroplating using the barrier metal 4 as a cathode and deposited through a photoresist pattern.
Metal leads are connected to the metal bumps as shown in FIG. 1B. Metal bumps 5 in FIG. 1B are disposed in the direction perpendicular to the sheet. A polyimide tape 6 has metal leads 7 formed thereon, being disposed along the tape in the same direction as that of the metal bumps 5. Each metal lead is laid upon a respective metal bump and they are pressed together and heated by a tool 8 so that they are bonded. If the metal bump 5 is coated with a plated Au layer and the metal lead 7 plated Sn layer, the heating causes Au--Sn eutectic alloy to be made, which gives a high bonding strength.
Such method involves problems as follows:
(1) Since the barrier metal is a multilayer, it is necessary to consider the bonding strength or barrier resistance between metal layers. If the bonding strength is low, a force applied to the metal lead easily causes peeling off between the metal layers or between the barrier metal and the bump. A large barrier resistance greatly affects the electric properties of the semiconductor device.
(2) The process for making the metal bumps requires the broad and precise techniques such as the deposition of the metal layer, plating, etching of the metal layer and photo-etching, resulting in high manufacturing cost and low yield. Further it is difficult for an ordinary assembly system to perform such process because of the necessity of high technology and expensive equipment.
(3) Dangerous and injurious chemicals are used in the process of etching the barrier metal. This makes it necessary to consider environmental protection.
(4) There is the possibility that during the bonding of the lead to the bump, the produced eutectic falls on the passivation layer 2 and the layer is cracked due to the high temperature, causing the effect thereof to be lowered. It makes the reliability of the device low.